Jay Shah, MD

Scheers H, Jacobs L, Casas L, Nemery B, and Nawrot TS. Long-Term Exposure to Particulate Matter Air Pollution Is a Risk Factor for Stroke: Meta-Analytical Evidence. Stroke. 2015 

Particle pollution, also known as particulate matter (PM), is a mixture of solid particles and liquid droplets found in the air. Inhalable coarse particles (diameter between 2.5-10 micrometers) and fine particles (diameter less than 2.5 micrometers) pose the greatest threat to human health. Potential mechanism includes inducing chronic systemic oxidative stress and inflammation which in-turn may promote atherosclerosis at the vasculature level. Thus, it has been postulated that PM exposure is an under recognized stroke risk factor. Recent meta-analyses characterized the short-term effects of PM on stroke hospitalization and mortality. However, the long-term effects of PM are relatively less known. Therefore, the authors conducted a meta-analysis to quantify the association between stroke risk and long-term PM exposure. The authors reviewed the literature for original studies assessing the association of long-term exposure of PM10 and PM2.5 to stroke events or stroke mortality. 

In total, 20 publications were included with combined >10 million persons and >200,000 stroke events. The studies were fairly evenly balanced between North America, Europe, and Eastern Asia. Hazard ratio with 95% CI was 1.061 for stroke event secondary long-term PM10 exposure and concluded that there exists a positive association between risk of stroke and PM exposure with 2-21% excess risk. Expectedly, there was significant geographical variation.

There are several limitations to this study. Three Chinese and two Japanese studies were excluded from sensitivity analysis due to dissimilarity. This certainly can be confounding as the Chinese studies had demonstrated significantly high PM levels. Another potential confounding variable, as pointed out by the authors, is the possible inaccuracy in determining exposure levels. Authors of the individual trials used data from central monitor stations and made efforts to obtain data from stations in closest proximity to the individual. However, this strategy can be problematic and does not account for important variables such as occupation, location of employment, and time spent outdoors. Several studies also did not capture smoking status and socioeconomic status within their data.

Therefore, data from this meta-analysis should be interpreted with caution. However, this is not to suggest that PM concentrations are not a public health concern. Clearly, there are enough reasons to reduce air pollution ranging from practical purposes (i.e. to improve visibility) to ecological reasons (i.e. to hasten the process of global warming). Furthermore, there is more robust evidence of harmful effects of particle pollution to pulmonary disease. Therefore, policy to decrease particle pollution will clearly be beneficial and may also decrease cardiovascular disease as this meta-analysis suggest.